1. Technical Field of the Invention
The present invention relates to an apparatus for controlling the drive of a generator mounted on a vehicle in order to regulate an output voltage of the generator, and in particular, to the apparatus in which control is made on an exciting current to flow through an exciting winding of the generator.
2. Related Art
Various types of apparatuses for controlling the drive of on-vehicle generators have been known. One type of such control apparatus is proposed by Japanese Patent Laid-open (unexamined) Publication No. 2003-284257 and U.S. Pat. No. 6,900,618 B2. According to these publications, the control apparatus detects an excitation current and the number of engine revolutions, calculates a generation torque, and controls an exciting current so that deviations in the generation torque is kept under a predetermined value. This control allows the revolutions of the engine to be kept in a stable state thereof.
However, in this control apparatus, the exciting current is detected by monitoring a voltage across a resistor connected in series to the exciting winding during a period during which a transistor for driving the exciting current to control a current flow through the exciting winding is in on state. In cases where a large power-consuming electric load, which has been electrically connected to an on-vehicle battery system (that is, to the on-vehicle generator), is electrically disconnected from the battery system, the period during which the above transistor is in an off state becomes longer. The longer such a period during which the transistor is in an off state, the more difficult the exciting current detected in real time. Therefore, in such a case, the detection of the exciting current becomes inaccurate, causing various types of control on the detected exciting current to be done in an inaccurate manner.
Specifically, the detected exciting current is temporarily stored and, if the foregoing longer off state of the transistor occurs, the value of the exciting current, which was detected last time before the electric disconnection of the load, is subjected the control. The exciting current decreases depending on the time constant of the exciting winding. This decrease will thus increase a difference between values of the detected and actual exciting currents, thereby decreasing accuracy in detecting the exciting current. As a result, the detected exciting current will not lead to an accurate calculation of a torque to be generated, with the result that the engine cannot be controlled properly.
A technique for preventing such a decrease in the accuracy in detecting the exciting current is provided, as a vehicle generation control apparatus, by Japanese Patent Laid-open Publication No. 2004-208342 (corresponding to U.S. Patent Application Publication No. 2004/0135374A1). This publication provides the technique that the duty of an exciting current for driving is kept at a minimum value for a predetermined period of time, even when a large capacity electric load is electrically disconnected from the battery system.
In the above vehicle generation control apparatus, the minimum duty is set to, for example, a predetermined value of 3% or more, which allows the exciting current to be detected by an exciting current detector. This setting thus makes it possible to detect the value of the actual exciting current at intervals with an improved accuracy.
However, the above minimum-duty technique is confronted with another drawback. That is, since the vehicle control generation apparatus is obliged to maintain the exciting current at a minimum duty or more at any time, the output voltage of the vehicle generator will increase to invite an excessive voltage state when being rotated at a high speed. Alternatively, it may take time until the output voltage decreases below a predetermined regulated voltage.